This invention relates to semiconductor devices such as a high electron mobility transistor (HEMT) and metal semiconductor field effect transistor (MESFET), and particularly to those having a nitride semiconductor region on a substrate of silicon or a silicon compound.
The HEMT and MESFET have been known which employ gallium-nitride-based compound semiconductors. Generally, such semiconductor devices had long been formed by creating a gallium-nitride-based semiconductor region on a sapphire substrate via a buffer region. However, besides being expensive, sapphire is so hard that the substrates made therefrom were difficult of dicing. These shortcomings of the sapphire substrate are absent from the substrates that are currently being made from silicon or a silicon compound (hereinafter collectively referred to as the silicon substrate) according to the teachings of Japanese Unexamined Patent Publication No. 2003-59948.
The semiconductor devices of this improved kind proved to possess their own difficulty: They tend to become electrically unstable in the event of potential fluctuations on the back of the silicon substrate in the course of their operation. A known remedy to this difficulty is the creation of an additional electrode on the back of the silicon substrate. Electrically coupled to the source or drain electrode of the device, the back electrode has its potential fixed to that of the source or drain electrode. A drain-source voltage is then applied between the front of the gallium-nitride-based semiconductor region and the back of the silicon substrate.
A problem left unresolved with such semiconductor devices arose from a great difference in linear expansion coefficient between the silicon substrate and the gallium-nitride-based semiconductor region. The difference is such that the semiconductor wafer would suffer a substantive warpage if the gallium-nitride-based semiconductor region were epitaxially grown on the silicon substrate to a thickness in excess of a certain limit. Not only would the wafer warpage adversely affect the electrodes subsequently fabricated on the semiconductor region, but this semiconductor regions itself might be cracked or otherwise impaired, with consequent deterioration of the electrical characteristics of the resulting devices. These deficiencies make it necessary to limit the thickness of the gallium-nitride-based semiconductor region to three micrometers or so even in cases where the multilayered buffer region, taught by the aforesaid unexamined Japanese patent application, is interposed between that region and the silicon substrate.
The gate, drain, and source electrodes are usually arranged side by side on the surface of the gallium-nitride-based semiconductor region in both HEMT and MESFET. The antivoltage strengths between these electrodes may be made sufficiently high for all practical purposes by provision of appropriate spacings therebetween. However, the same spatial scheme is not applicable to the enhancement of antivoltage strength between the surface of the gallium-nitride-based semiconductor region and the back electrode on the silicon substrate because of the noted limitations imposed upon the thickness of the semiconductor region.
It might be contemplated to make the silicon substrate higher in resistance for a greater voltage-withstanding capability. This solution is impractical because the resistivity of the silicon substrate is 1000 ohms per centimeter at most. The silicon substrate is also desired to be as thin as feasible from the standpoint of heat radiation, another reason why the silicon substrate is per se not adaptable for improvement of antivoltage strength. The same holds true with substrates made from silicon carbide or other silicon compounds.